Techniques for converting the number of scanning lines of a television signal are known, as disclosed, for example, in the Laid-Open Japanese Patent Sho.54-138325 and the Published Japanese Patent Hei.4-3151.
FIG. 1 shows such a conventional scanning line conversion circuit, as disclosed in the Published Japanese Patent Hei.4-3151. This conventional circuit receives an interlace scanning signal at its input, and then converts it to a progressive scanning signal. The circuit generates a progressive scanning signal having scanning lines which are twice that of the input interlace scanning signal, by compressing the time axes of the interlace scanning signal and an interpolated signal obtained from the interlace scanning signal. The interpolated signal is obtained by adapting a motion. The interpolated signal production mode has a still picture mode and a moving picture mode. In the still picture mode, the interpolated signal is produced by an inter-field image processing for corresponding scanning line signals in two adjacent fields. In the moving picture mode, the interpolated signal is produced by an intra-field image processing for two adjacent scanning line signals in the same field. The inter-field image processing and the intra-field image processing are selectively implemented in response to a motion signal detected by a motion detector.
In FIG. 1, an input signal is placed on input terminal 312 and is then supplied to field memory 315 via field memory 314. The input signal and the output of field memory 315 are supplied to field memory 315. The output of subtractor 319 is also supplied to coefficient multiplier 322 as a still picture signal for use in the inter-field image processing. Subtractor 319 outputs a motion signal. This motion signal controls coefficient multipliers 321 and 322 via motion detector 320. The output of field memory 314 is supplied to line memory 316. The output of field memory 314 is also supplied to adder 317. Adder 317 adds the outputs of line memory 316 and field memory 314 together. The output of adder 317 is then supplied to 1/2 coefficient multiplier 318. The output of coefficient multiplier 318, which represents an intra-field interpolation signal effective for moving pictures, is supplied to adder 323 via coefficient multiplier 321. Adder 323 combines the outputs of coefficient multiplier 321 and coefficient multiplier 322, respectively representing the motion picture and the still picture. The output of adder 323 is then supplied to time compressor 325. The output of time compressor 325 is supplied to selector 326. Selector 326 also receives another time compression signal from time compressor 324. The time compressor 324 is directly coupled to field memory 314, so that the time compression signal from time compressor 324 represents the original interlaced scanning signal on input terminal 312. Selector 326 alternately selects the original interlaced signal and its line interpolation signal. According to the conventional system as described above, coefficient multiplier 321 is supplied with the intra-field image processing signal, while coefficient multiplier 322 is supplied with the inter-field image processing signal. Coefficient multipliers 321 and 322 are, however, commonly controlled by motion detector 320.
In the conventional system as described above, the inter-field image processing, with respect to still pictures, can be effectively executed thus improving a vertical resolution of images and eliminating inter-line flickers of images. However, in its intra-field image processing, with respect to moving pictures, the quality of the moving picture is inferior to that of the still pictures due to the fact that the processing of the moving pictures are executed within a same field. For example, in the NTSC system the number of effective scanning lines is about 480 lines per frame (or 240 lines per field). Consequently, as to still pictures, the inter-field image processing can be carried out for all of the 480 scanning lines in one frame. Therefore a vertical resolution around 480 lph (line per screen height) is obtained. On the other hand, since moving pictures are processed by the intra-field image processing, information on merely 240 scanning lines is used. Therefore, the vertical resolution for the moving pictures is principlely limited to 240 lph. Further, since the image processing is based on existing filters the practical resolution is further reduced. According to the image processing by the conventional circuit as shown in FIG. 1, the filter for intra-field image processing has a square-cosine characteristic of 0 dB at the vertical spatial frequency of 0 lph, -6 (minus 6) dB at 240 lph and -.infin. (minus infinity) dB at 480 lph.
This characteristic means as follows. Firstly, since the frequency response characteristics of the filter passing band is not flat, signal components in the vicinity of 240 lph are attenuated. This frequency response characteristics has a great influence on vertical image sharpness. That is, a blurring of images increases so that the picture quality deteriorates significantly. Secondly, vertical high frequency components at or above the 240 lph, which have not been completely attenuated in the filter, act as aliasing noises causing the picture quality to be significantly deteriorated. Therefore, in the conventional circuit a relatively high quality image can be achieved for the still picture. However, since image processing for the moving picture is insufficient in comparison to that of the still pictures a great difference occurs between the picture qualities in the still pictures and the moving pictures, thus resulting a very unnatural image in the motion adaptive image processing.
In the conventional television signal processor for converting the number of scanning lines, a high quality image can be obtained with the still picture, but when the motion adaptive converting operation is executed, a great picture quality difference between the still picture and the moving picture occurs due to the insufficiency of the picture quality of the moving picture, as described above. Therefore, this results in very unnatural images.